A label won't update if dropdown is inside updatepanel

The SelectedIndexChanged event fires properly and execute the code without any errors. The last thing I do in it is to set the value of a label which isn't updated. If I move the TD element inside updatepanel then it will refresh the label but screen flashes and I am trying to avoid the screen flash.

<td style="width:10%;"><asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:DropDownList ID="ddlF1" runat="server" Width="95%" AutoPostBack="true" OnSelectedIndexChanged="ddlF1_SelectedIndexChanged"></asp:DropDownList></ContentTemplate></asp:UpdatePanel></td><td style="width:10%;"><asp:UpdatePanel ID="UpdatePanel5" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:DropDownList ID="ddlF2" runat="server" Width="95%" AutoPostBack="true"  OnSelectedIndexChanged="ddlF2_SelectedIndexChanged"></asp:DropDownList></ContentTemplate></asp:UpdatePanel></td>

The code below does update the label:

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"><ContentTemplate><td style="width:10%;"><asp:DropDownList ID="ddlF1" runat="server" Width="95%" AutoPostBack="true" OnSelectedIndexChanged="ddlF1_SelectedIndexChanged"></asp:DropDownList></td></ContentTemplate></asp:UpdatePanel><asp:UpdatePanel ID="UpdatePanel5" runat="server" UpdateMode="Conditional"><ContentTemplate><td style="width:10%;"><asp:DropDownList ID="ddlF2" runat="server" Width="95%" AutoPostBack="true"  OnSelectedIndexChanged="ddlF2_SelectedIndexChanged"></asp:DropDownList></td></ContentTemplate></asp:UpdatePanel>

The only code I have in ddlF1 and ddlF2's SelectIndexChange is  lblTemp = "My Test";